v8js is a new PHP extension able to run JavaScript code inside V8, Google's JavaScript interpreter that powers for example Chrome and NodeJS. This extension is highly alpha - and its API would probably change in the months ahead. Since documentation is lacking, I invite you to repeat the discovering process I follow in this post in case you find some differences in a new version of v8js.

He gives you the (PECL-based) commands to get the extension and it's needed dependencies installed and enabled. He uses PHP's own Reflection features to look at the extension and find its methods including: "executeString", "getPendingException" and "getExtensions".
To show it in action, he implements an old standby to test new languages - the FizzBuzz example - in Javascript, executed inside the PHP. He also includes a quick example of how to load in an external Javascript file and execute the results.